A problem in the technology of detecting the direction of electric power flow is to obtain a fast response time in situations where the direction of power flow may reverse quickly. For example, in a variable frequency alternating current (AC) motor drive system conditions where the motor begins to overhaul, that is to generate electric power, can develop quickly. And when the motor begins to overhaul the electric power generated by the motor must be absorbed by the motor drive system in order to maintain smooth functioning of the system. For example, if power generated by the motor is not absorbed, bus voltages may rise beyond the capability of components, leading to component failure. The reversal of electric power flow must trigger switching activities in order that the electric power generated by the motor be absorbed.
Methods used in the past to detect the direction of electric power flow include detecting a bus voltage. Generally bus voltage tends to rise as electric power is generated by a load, particularly when the load usually absorbs electric power. A disadvantage of using bus voltage rise for detecting a rerversal of electric power flow is that the rise in voltage is a much slower process than is the reversal of electric power flow. Factors causing bus voltage to rise slowly include the use of filters to smooth out unwanted voltage fluctuations on the bus. And the filters naturally slow the rate of bus voltage rise in response to disturbances such as a change in the direction of electric power flow.
For example, in a variable frequency AC inverter used to drive an AC motor the inverter often draws its source of electric power from a direct current (DC) bus. A capacitor is connected across the bus as a filter in order to smooth out unwanted voltage fluctuations in the bus voltage. When a motor being driven by the inverter goes into overhauling conditions, then the electric power generated by the motor is rectified and fed back to the DC bus thereby charging the filter capacitor and causing the bus voltage to rise. However, the time required for the bus voltage to rise will be long when compared to the time required for the direction of electric power flow to reverse because the reversed electric power flow must first charge the capacitor in order to raise the bus voltage. In summary, bus voltage is a delayed indicator of electric power flow direction change.
Electronic power flow meters designed in the past have been either unsuitable or inconvenient for detecting rapid change in the direction of electric power flow. Examples of such electronic power flow meters are given in U.S. Pat. No. 4,217,545 issued to Kusui et al on Aug. 12, 1980, and U.S. Pat. No. 3,875,509 issued to Milkovic on Apr. 1, 1975, and U.S. Pat. No. 4,224,671 issued to Sugigama on Sept. 23, 1980. When electronic power flow meters such as disclosed in U.S. Pat. No. 3,875,509 are used in circuits using pulse width modulated (PWM) techniques, such as a voltage source PWM inverter, the time division multiplier clocks must be operated at inconveniently high clock rates. The clock rates must be well in excess of the inverter PWM carrier frequency to assure a reasonable measurement of the power flow to or from the inverter output. Expensive hardware must be used to achieve even modest measurement accuracy. Older mechanical type watt meters of the reversible drive type are too slow to be useful in detecting reversal in direction of electric power flow.